1. Field
This application relates to technology for non-volatile storage.
2. Description of the Related Art
A variety of materials show reversible resistance-switching behavior, and as such may be suitable for use with memory systems. For example, transition metal oxides have been proposed for reversible resistance-switching memories. Upon application of sufficient voltage, current, or other stimulus, the reversible resistance-switching material switches to a stable low-resistance state, which is sometimes referred to as SETTING the device or performing a SET operation. This resistance-switching is reversible such that subsequent application of an appropriate voltage, current, or other stimulus can serve to return the reversible resistance-switching material to a stable high-resistance state, which is sometimes referred to as RESETTING the device or performing a RESET operation. This conversion can be repeated many times. The low resistance state is sometimes referred to as an “on” state. The high resistance state is sometimes referred to as an “off” state. For some reversible resistance-switching materials, the initial state is low-resistance rather than high-resistance. For purposes of the following discussion, any of the operations of RESETTING and SETTING may be considered to be a programming operation. In some devices, prior to being able to perform SET operations and RESET operations, a devices must be initialized by applying a voltage potential across the reversible resistance-switching material in an operation referred to as FORMING.
These reversible resistance-switching materials are of interest for use in nonvolatile memory systems. One type of memory system is referred to as a cross-point array, which is a matrix of memory elements typically arranged along x-axes (e.g., word lines) and along y-axes (e.g., bit lines). A digital value may be stored as a memory resistance (high or low). The state of a memory cell can be read by supplying appropriate voltages to the bit line and word line connected to the selected memory cell. The state of the memory cell can be read as an output voltage of the bit line connected to the selected memory cell. One resistance state may correspond to a data “0,” for example, while the other resistance state corresponds to a data “1.” Some switching materials may have more than two stable resistance states.
Operating memory devices that employ reversible resistance-switching materials is challenging.